Canon EOS 70D Instruction Manual page 54

Before You Start
Format the card in the following cases:
The card is new.
The card was formatted by a different camera or a computer.
The card is full with images or data.
A card-related error is displayed.
Low-level Formatting
Perform low-level formatting if the card's recording or reading speed seems
slow or if you want to erase all data on the card.
Since low-level formatting will format all recordable sectors on the card, the
formatting will take slightly longer than normal formatting.
You can stop the low-level formatting by selecting [Cancel]. Even in this
case, normal formatting will be completed and you can use the card as
usual.
When the card is formatted or data is erased, only the file management
information is changed. The actual data is not completely erased. Be
aware of this when selling or discarding the card. When discarding the
card, execute low-level formatting or destroy the card physically to
prevent the personal data from being leaked.
Before using a new Eye-Fi card, the software on the card must be
installed in your computer. Then format the card with the camera.
The card capacity displayed on the card format screen may be smaller
than the capacity indicated on the card.
This device incorporates exFAT technology licensed from Microsoft.
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